This paper introduces the design data and numerical analysis of a braceless steel semi-submersible wind turbine. The hull of the semi-submersible wind turbine is designed to support a reference 5-MW horizontal axis wind turbine at a site in the northern North Sea. The hull is composed of a central column, three side columns and three pontoons. The side columns and pontoons are arranged radially outward from the central column which is used to support the wind turbine. The side columns form the corners of a triangle on the horizontal plane and are connected by the pontoons to the central column at the bottom to form an integrated structure. Numerical analysis has been carried out to analyze the intact stability, natural periods and modes and global dynamic responses in winds and waves. Results of the numerical analysis show that the design has very good intact stability, well designed natural periods and modes, moderate rigid-body motions in extreme environmental conditions and a reasonable structural design. This paper emphasizes the structural responses of the hull considering both the global and local load effects. The global forces and moments in the hull are calculated by carrying out time-domain global analysis and used as inputs for simplified ultimate limit state design checks for structural strength of the hull. The design can be used as a reference semi-submersible wind turbine.

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